Existing projection systems equipped with three micro mirror devices make extensive use of so called Philips prism assemblies to illuminate the micro-mirror devices and to combine the fundamental colours reflected by the micro-mirror devices.
Prism assemblies in these projectors must be designed carefully to prevent light not selected to be projected (i.e. light reflected by pixels in another state than the ON state as well as light reflected by edges and surfaces next to and under the pixels of the micro-mirror devices) to ruin the contrast of the projection system.
One type of micro mirror device is an FTP DMD, in the micro-mirrors can be tilted by +/−12 degrees around a diagonal. The illumination light must come from a direction perpendicular to this tilt axis and this light is parallel to the pixel's diagonal or at 45 degrees with respect to the DMD's horizontal pixels rows. The light reaches the micro-mirrors at an incidence angle of 24 degrees. Depending on the micro mirror tilt angle (−12, 0, +12 degrees respectively), the illumination light is reflected into specific directions corresponding to an ON state, a FLAT (rest) state and an OFF state respectively.
The illumination light cone and the ON state light cone are usually separated from each other by means of a TIR (total internal reflection) prism assembly. In the case of a 3 chips projector, the colour splitting and recombining Philips trichroic prism assembly is inserted between the TIR prism and the 3 DMD's.
The FLAT state light cone and the OFF state light cone are usually separated from the ON state light cone by a suitable aperture in the projection lens.
A new type of DMD spatial light modulator called TRP or “Tilt and Roll Pixel” has been developed having a movement of micro mirrors (the “pixels”) is different than in the case of a classical FTP DMD. In a TRP DMD the movement of the individual micro mirrors is more complex and may be seen as a compound rotation around two perpendicular axes. The FLAT or rest state is obviously the same as for a classical FTP DMD, but the ON and OFF states are different. In the ON state, the micro mirror ends up in a position that can be approximated as being the result of a tilt around one of the (square) pixel median lines (horizontal or vertical). In the OFF state light, the micro mirror ends up in a position that can be approximated as being the result of a tilt around the other (square) pixel median line.
With the arrival of the new type of micro mirror devices, where a new type of micro-mirror known as Tilt-and-Roll Pixel (TRP) is used, it has become even more challenging to design prism assemblies that will efficiently isolate the light reflected by ON pixels from unwanted light reflected by the micro mirror devices.
In the case of a 3 chips projector, the Philips prism architecture is a very popular one, but its use with TRP DMD's raises additional problems as well:                The incidence angle of the illumination light cone must now be 34 degrees (in air) instead of 24 degrees. As a consequence, the “dichroic shift” phenomenon, responsible of big light losses in the Philips prism assembly, and caused by the different incidence angle, onto the dichroic mirrors, for the illumination light and the ON state light will be even more severe with TRP chips than with FTP chips.        For the same reason, a redesign of the TIR prism will be necessary (other angles), unevitably leading to a bigger assembly and thus to a longer backworking distance for the projection lens (increasing the PJ lens cost).        
U.S. Pat. No. 7,207,678 B2 describes a dual TIR prism for better OFF state light removal, which is limited to single chip architectures and FTP DMD's. The description in U.S. Pat. No. 7,207,678 of a 3 chips engine includes a Philips prism and only one illumination TIR prism in front of the colour splitting. There is also no adaptation foreseen for application of a dual TIR prism on a recombination X-cube nor for the fast OFF state light removal in the case of TRP DLPs.